Sensors in the sense of this invention serve for ascertaining process variables in liquids, especially fresh- and, industrial water, as well as in gases. Examples of typical process variables are turbidity, solids content and sludge level. Forms of embodiment can, however, concern other chemical and physical, process variables. To be mentioned here are determining nitrate content, UV absorption, pressure measurement or contactless fill level measurement, especially by ultrasound. Measuring devices suitable for determining the corresponding process variables are available from the group of firms, Endress+Hauser, in a large number of variants.
Usually, the sensors are arranged in a sensor body. Many of the mentioned process variables are optically determined. In such case, electromagnetic waves of a certain wavelength are received through an optical window in the sensor body. Known from DE 42 33 218 C2 is an apparatus for turbidity measurement, wherein, supplementally, an optical source is provided, which is connected with the sensor body via another optical window.
Known from EP 1 816 462 B1 is an arrangement, in the case of which only one window is provided for source and receiver.
If the process variables are non-optically determined, the sensor contacts the medium via a membrane or a corresponding adapting layer.
Through operation in aqueous or gaseous media, especially also wastewater, scale formations, fouling, deposits and accretions occur on the optical window or the membrane, whereby measurement results are corrupted or the measurement made impossible. Known from the brochure “Technical information TurbiMax W CUS41/CUS41-W” of the group of firms, Endress+Hauser, is that windows can be cleaned by a wiper mounted on the end plate of the sensor body.
The wiper is driven by a motor in the sensor body, wherein the movement is transmitted by means of a shaft. It must be heeded that the position where the shaft passes through the sensor body is sealed stably over the long term, in order that no medium can penetrate into the sensor body.
The movement of the wiper is controlled by a control unit and occurs alternately in, and counter to, the clockwise direction.
For preventing defective measurements, after ending the cleaning, the wiper must be brought into a defined resting position, which is positioned far enough removed from the windows. If the wiper is too near to the measuring apparatus, the measurement is disturbed and corrupted by the wiper.